I think it's limited to the ones listed in the board selection pull down list in the IDE. This has increased over time with the IDE version upgrades. The file boards.txt in the hardware folder lists the devices supported in more detail I believe.

you can see that atmega1280 is supported. You can even download a .hex file for it there. I guess this is the chip that is being used on the new Arduino Mega... but I am more into the opposite direction (use smaller attiny mcus) so I guess you have to try it because I never worked with the atmega1280.